Support devices that are the subject of the invention, are e.g. ‘oil & gas’ structures and wind turbine structures. These prior art structures can be roughly divided into two parts, i.e. a superstructure and a substructure or foundation. The superstructure forms the topside, i.e. in ‘oil & gas’ platforms or the ‘rotor and tower’ in wind turbines. The substructure (or foundation) typically are jackets in ‘oil and gas’ platforms, and for wind turbines, the substructure is formed by monopiles, triples, tripods or jackets.
There are several alternative designs of substructures (foundations) for ‘oil & gas’ and wind turbines: monopoles, jackets, more particularly jackets with piles through the legs and jackets with skirt piles, triples and tripods. The latter substructures are normally fixed to the soil by other tubular elements—piles—which secure the soil-substructure interactions.
A monopile substructure normally comprises the pile itself and a transition piece on top, also a tubular element.
Forces (Load) loading the substructure—the superstructure (rotor for turbines/topside for ‘oil and gas’), waves and current—are transferred to the substructure, and through the piles to the soil by means of friction forces on the (shaft) pile outside and the plug inside in the case the pile is open. The forces from the plug are further transferred to the soil through end-bearing forces.
With piles closed on the bottom side, there are no friction forces on the inside and the only soil reactions are the friction forces on the outside of the shaft and the (end)-bearing forces on the tip.
Substructures are subjected to large Ultimate Limit States (ULS)) and extreme (ExtremeLS, hurricanes) static compression loads (in the case of ‘oil and gas’) or large bending loads (in the case of wind turbines), which requires additional material in the structure to achieve sufficient strength and not to lose stability of the form (i.e. no occurrence of buckling).
Furthermore, substructures are also dynamically loaded in a broad frequency band created by waves and wind, which inevitably gets very close to the natural frequency of the substructure itself and the dynamic response (Dynamic Amplification Factor (DAF)) is very high, thus, resulting in very high loads. The damping of the substructures is normally very low (structural damping only) which means that the load is not reduced significantly and remains high. This high dynamic loading also requires the use of more material to achieve the required lifetime (fatigue) of the substructure.
The majority of piles are driven open ended which means that the interaction with the soil is mostly through friction forces. These friction forces are strongly dependent on the vibrations of the substructures. Because the vibrations are a lot, the pile has to be long (beneath the mudline) and/or with a larger diameter. On the other hand, the friction limit of the soil is 100 kPa, while the end-bearing limit of the pile (which is not used) is 10 MPa thus 100 times lower and this means that piles that primarily utilize friction have to be longer. A further disadvantage of the open piles is that the friction damping (currently used) is 2 times lower that then the end-bearing damping of the soil, which isn't used. This means that either longer piles or additional dampers in the super/substructure are needed.
Piles are very rarely driven in the soil with a closed bottom end because this requires much more energy and an increased wall thickness. Once driven open ended, the end-bearing capacity of these piles is not utilized. As a result, substructures for large loads and loads with a wide frequency spectrum tend to be heavy and expensive.
There is a need to improve prior art substructures with respect to their weight, strength, stiffness, stability and buckling safety, i.e. to provide an improved dynamic behavior and pile capacity.